1. Field of the Invention
The present invention relates to an alignment method suitable for establishing alignment between each of a plurality of shot areas defined on a photosensitized substrate and a mask pattern to be printed on each of the shot areas in a lithographic process for fabrication of semiconductor devices, liquid crystal display devices, and others. The present invention also relates to an exposure apparatus for use in such an alignment method.
2. Related Background Art
In general, the fabrication of typical semiconductor devices involves the formation of multilayer circuit patterns on a semiconductor wafer (simply referred to as "wafer" hereinafter). This requires that the registration between circuit patterns of related layers (such as two adjacent layers) on the wafer should be within a specified tolerance in order to ensure that the fabricated semiconductor devices meet design requirements. As semiconductor devices have developed to have a greater and greater number of components per IC chip, leading to LSIs (large-scale-integrated circuits) and then to VLSIs (verylarge-scale-integrated circuits), the registration accuracy required by the lithographic process involved in the fabrication of semiconductor devices has become more and more stringent.
In the lithographic process for fabricating IC chips, there have been widely used step-and-repeat type of projection exposure apparatuses (called "steppers"). Where the stepper is used, the exposure region of a wafer is divided into a number of shot areas, and a pattern formed on a reticle (or mask) is successively printed on the shot areas with the printed pattern aligned (or overlaid) with the previously defined patterns on the wafer. More recently, so-called step-and-scan type of projection exposure apparatuses have been developed in order to enable the exposure of a larger pattern formed on a larger reticle without necessitating the use of a projection optical system having a larger diameter. In the step-and-scan type of projection exposure apparatus, each shot area on the wafer is positioned at the scan starting position by a stepping operation, and then the reticle and the shot area are moved in synchronism with each other and relative to the projection optical system, during which exposure is performed, so that a scanning exposure is performed with respect to each shot area.
In these types of projection exposure apparatuses, a laser interferometer is used to perform precision measurement of the position of a wafer stage carrying a wafer. In addition, an optical alignment sensor is used to determine the position of an alignment mark (wafer marks) formed for each shot area on the wafer as, for example, a relief pattern. The position of the wafer stage is controlled based on the measured values from the laser interferometer as well as the determination results from the optical alignment sensor.
The types of projection exposure apparatuses described above can provide the registration accuracy required for the fabrication of semiconductor devices whose complexity corresponds to that of a typical 256-megabit DRAM (dynamic-random-access-memory) chip. However, it is anticipated that higher registration accuracy tolerating, for example, errors of less than 50 nm (nanometers) with a minimum width of ruling of 150 nm may be required in the future for the fabrication of semiconductor devices whose complexity corresponds to that of a possible 1-gigabit DRAM chip. In order to achieve this registration accuracy by using any of the existing types of projection exposure apparatuses, the measurement accuracy of the laser interferometer must be as high as that tolerating errors of less than several nanometers, and a system including the wafer stage must be sufficiently stable to prevent any significant variation in the relative position between the wafer and the moving mirror monitored by the laser interferometer. In addition to such a higher registration requirement, there is another requirement that high throughput (i.e., the number of wafers that can be processed per unit of time) be achieved. Unfortunately, it is difficult to meet both of these requirements with any of the existing alignment techniques.
In view of the foregoing, it is an object of the present invention to provide an alignment method which can achieve higher registration accuracy without any need for high stability in the position control of the stage used for positioning the photosensitized substrate nor any need for tight tolerance of the measurement means (such as a laser interferometer) used for measuring the position of the stage, and even when the positioning is performed at a higher speed.
It is another object of the present invention to provide an exposure apparatus usable for performing such an alignment method.